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Major progress on Gradle port

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Complete:
--------
- src/* documentation resources moved to 'docs' subproject

- docbook sources upgraded to Docbook 5

- formatted all docbook sources to strip tab characters and
  eliminate trailing whitespace

- all projects compile and test successfully

- all artifacts upload successfully to s3, static.sf.org, etc.

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- documentation L&F needs work. CSS, images, and highlighting aren't
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- spring-integration-jdbc codegen bits in Maven POM need to be
  transcribed into gradle

- dependencies that were optional or provided scope in maven are
  currently 'compile' scope in Gradle.  Need to figure out support
  in Gradle to fix this.

- run through Eclipse classpath and project generation scenarios

- delete all Maven artifacts
This commit is contained in:
Chris Beams
2010-10-26 18:43:14 -04:00
parent d677da9c62
commit 677fca51a9
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<?xml version="1.0" encoding="UTF-8"?>
<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="gateway"
xmlns:xlink="http://www.w3.org/1999/xlink">
<title>Inbound Messaging Gateways</title>
<section id="gateway-proxy">
<title>GatewayProxyFactoryBean</title>
<para>
Working with Objects instead of Messages is an improvement. However, it would be even better to have no
dependency on the Spring Integration API at all - including the gateway class. For that reason, Spring
Integration also provides a <classname>GatewayProxyFactoryBean</classname> that generates a proxy for
any interface and internally invokes the gateway methods shown above. Namespace support is also
provided as demonstrated by the following example.
<programlisting language="xml"><![CDATA[<gateway id="fooService"
service-interface="org.example.FooService"
default-request-channel="requestChannel"
default-reply-channel="replyChannel"/>]]></programlisting>
Then, the "fooService" can be injected into other beans, and the code that invokes the methods on that
proxied instance of the FooService interface has no awareness of the Spring Integration API. The general
approach is similar to that of Spring Remoting (RMI, HttpInvoker, etc.). See the "Samples" Appendix for
an example that uses this "gateway" element (in the Cafe demo).
</para>
<para>
The reason that the attributes on the 'gateway' element are named 'default-request-channel' and
'default-reply-channel' is that you may also provide per-method channel references by using the
@Gateway annotation.
<programlisting language="java"><![CDATA[ public interface Cafe {
@Gateway(requestChannel="orders")
void placeOrder(Order order);
}]]></programlisting>
... as well as <code>method</code> sub element if yuo prefer XML configuration (see next paragraph)
</para>
<para>
It is also possible to pass values to be interpreted as Message headers on the Message
that is created and sent to the request channel by using the @Header annotation:
<programlisting language="java"><![CDATA[ public interface FileWriter {
@Gateway(requestChannel="filesOut")
void write(byte[] content, @Header(FileHeaders.FILENAME) String filename);
}]]></programlisting>
</para>
<para>
If you prefer XML way of configuring Gateway methods, you can provide <emphasis>method</emphasis> sub-elements
to the gateway configuration (see below)
<programlisting language="xml"><![CDATA[<si:gateway id="myGateway" service-interface="org.foo.bar.TestGateway"
default-request-channel="inputC">
<si:method name="echo" request-channel="inputA" reply-timeout="2" request-timeout="200"/>
<si:method name="echoUpperCase" request-channel="inputB"/>
<si:method name="echoViaDefault"/>
</si:gateway>]]></programlisting>
</para>
<para>
You can also provide individual headers per method invocation via XML.
This could be very useful if the headers you want to set are static in nature and you don't want
to embed them in the gateway's method signature via <classname>@Header</classname> annotations.
For example, in the Loan Broker example we want to influence how aggregation of the Loan quotes
will be done based on what type of request was initiated (single quote or all quotes). Determining the
type of the request by evaluating what gateway method was invoked, although possible would
violate the separation of concerns paradigm (method is a java artifact),  but expressing your
intention (meta information) via Message headers is natural in a Messaging architecture.
<programlisting language="xml"><![CDATA[<int:gateway id="loanBrokerGateway"
service-interface="org.springframework.integration.loanbroker.LoanBrokerGateway">
<int:method name="getLoanQuote" request-channel="loanBrokerPreProcessingChannel">
<int:header name="RESPONSE_TYPE" value="BEST"/>
</int:method>
<int:method name="getAllLoanQuotes" request-channel="loanBrokerPreProcessingChannel">
<int:header name="RESPONSE_TYPE" value="ALL"/>
</int:method>
</int:gateway>]]></programlisting>
In the above case you can clearly see how a different header value will be set for the 'RESPONSE_TYPE'
header based on the gateway's method.
</para>
<para>
As with anything else, Gateway invocation might result in errors.
By default any error that has occurred downstream will be re-thrown as a MessagingExeption (RuntimeException)
upon the Gateway's method invocation. However there are times when you may want to treat an Exception as a valid reply,
by mapping it to a Message. To accomplish this our Gateway provides support for Exception mappers via the
<emphasis>exception-mapper</emphasis> attribute.
</para>
<para>
<programlisting language="xml"><![CDATA[<si:gateway id="sampleGateway"
default-request-channel="gatewayChannel"
service-interface="foo.bar.SimpleGateway"
exception-mapper="exceptionMapper"/>
<bean id="exceptionMapper" class="foo.bar.SampleExceptionMapper"/>
]]></programlisting>
<emphasis>foo.bar.SampleExceptionMapper</emphasis> is the implementation of
<emphasis>org.springframework.integration.message.InboundMessageMapper</emphasis> which only defines one method: <code>toMessage(Object object)</code>.
<programlisting language="java"><![CDATA[public static class SampleExceptionMapper implements InboundMessageMapper<Throwable>{
public Message<?> toMessage(Throwable object) throws Exception {
MessageHandlingException ex = (MessageHandlingException) object;
return MessageBuilder.withPayload("Error happened in message: " +
ex.getFailedMessage().getPayload()).build();
}
}
]]></programlisting>
</para>
<para>
<important>
Exposing messaging system via POJO Gateway is obviously a great benefit, but it does come at the price so there
are certain things you must be aware of.
We want our Java method to return as quick as possible and not hang for infinite amount of time until they can
return (void , exception or return value). When regular methods are used as a proxies in front of the Messaging
system we have to take into account the asynchronous nature of the Messaging Systems. This means that there might
be a chance that a Message hat was initiated by a Gateway could be dropped by a Filter, thus never reaching a
component that is responsible to produce a reply. Some Service Activator method might result in the Exception,
thus resulting in no-reply (as we don't generate Null messages).So as you can see there are multiple scenarios
where reply message might not be coming which is perfectly natural in messaging systems. However think about the
implication on the gateway method.  The Gateway's method input arguments  were incorporated into a Message and
sent downstream. The reply Message would be converted to a return value of the Gateway's method. So you can see
how ugly it could get if you can not guarantee that for each Gateway call there will alway be a reply Message.
Basically your Gateway method will never return and will hang infinitely. (work in progress!!!!)
One of the ways of handling this situation is via AsyncGateway (explained later in this section). Another way of handling it is to explicitly set the reply-timeout attribute. This way gateway will not hang for more then the time that was specified by the reply-timout and will return 'null'. 
</important>
</para>
</section>
<section id="async-gateway">
<title>Asynchronous Gateway</title>
<para>
As a pattern the Messaging Gateway is a very nice way to hide messaging-specific code while still exposing the full capabilities of the
messaging system. And <classname>GatewayProxyFactoryBean</classname> provides a convenient way to expose a Proxy over a service-interface
thus giving you a POJO-based access to a messaging system (based on objects in your own domain, or primitives/Strings, etc).  But when a
gateway is exposed via simple POJO methods which return values it does imply that for each Request message (generated when the method is invoked)
there must be a Reply message (generated when the method has returned). Since Messaging systems naturally are asynchronous you may not always be
able to guarantee the contract where <emphasis>"for each request there will always be be a reply"</emphasis>. 
With Spring Integration 2.0 we are introducing support for an <emphasis>Asynchronous Gateway</emphasis> which is a convenient way to initiate
flows where you may not know if a reply is expected or how long will it take for it to arrive.
</para>
<para>
A natural way to handle these types of scenarios in Java would be relying upon <emphasis>java.util.concurrent.Future</emphasis> instances, and
that is exactly what Spring Integration uses to support an <emphasis>Asynchronous Gateway</emphasis>.
</para>
<para>
From the XML configuration, there is nothing different and you still define <emphasis>Asynchronous Gateway</emphasis> the same way as a regular Gateway.
<programlisting language="xml"><![CDATA[<int:gateway id="mathService" 
service-interface="org.springframework.integration.sample.gateway.futures.MathServiceGateway"
default-request-channel="requestChannel"/>]]></programlisting>
However the Gateway Interface (service-interface) is a bit different.
<programlisting language="java">public interface MathServiceGateway {
Future&lt;Integer&gt; multiplyByTwo(int i);
}</programlisting>
</para>
<para>
As you can see from the example above the return type for the gateway method is <classname>Future</classname>. When
<classname>GatewayProxyFactoryBean</classname> sees that the
return type of the gateway method is <classname>Future</classname>, it immediately switches to the async mode by utilizing
an <classname>AsyncTaskExecutor</classname>. That is all. The call to a method always returns immediately with <classname>Future</classname>
encapsulating  the interaction with the framework.
Now you can interact with the <classname>Future</classname> at your own pace to get the result, timeout, get the exception etc...
<programlisting language="java">MathServiceGateway mathService = ac.getBean("mathService", MathServiceGateway.class);
Future&lt;Integer&gt; result = mathService.multiplyByTwo(number);
// do something else here since the reply might take a moment
int finalResult =  result.get(1000, TimeUnit.SECONDS);</programlisting>
For a more detailed example, please refer to the <emphasis>async-gateway</emphasis> sample distributed within the Spring Integration samples.
</para>
</section>
<section>
<title>Gateway behavior when no response is coming</title>
<para>
As it was explained earlier, Gateway provides a convenient way of interacting with Messaging system via POJO method
invocations, but realizing that a typical method invocation, which is generally expected to always return (even with Exception),
might not always map one-to-one to message exchanges (e.g., reply message might not be coming which is equivalent to
method not returning), it is important to go over several scenarios especially in the Sync Gateway case and understand
what the default behavior of the Gateway and how to deal with these scenarios to make Sync Gateway behavior more
predictable regardless of the outcome of the message flow that was initialed from such Gateway.
</para>
<para>
There are certain attributes that could be configured to make Sync Gateway behavior more predictable,
but some of them might not always work as you might have expected. One of them is <emphasis>reply-timeout</emphasis>.
So, lets look at the <emphasis>reply-timeout</emphasis> attribute and see how it can/can't influence the behavior
of the Sync Gateway in various scenarios. We will look at single-theraded scenario
(all components downstream are connected via Direct Channel) and multi-theraded scenarios
(e.g., somewhere downstream you may have Pollable or Executor Channel which breaks single-thread boundary)
</para>
<para>
<emphasis>Long running process downstream</emphasis>
</para>
<para>
<emphasis>Sync Gateway - single-threaded</emphasis>.
If a component downstream is still running (e.g., infinite loop or a very slow service), then setting <emphasis>reply-timeout</emphasis>
has no effect and Gateway method call will not return until such downstream service exits (e.g., return or exception).
<emphasis>Sync Gateway - multi-threaded</emphasis>.
If a component downstream is still running (e.g., infinite loop or a very slow service), in a multi-threaded message
flow setting <emphasis>reply-timeout</emphasis> will have an effect by allowing gateway method invocation to
return once the timeout has been reached, since <classname>GatewayProxyFactoryBean</classname>  will simply
poll on the reply channel waiting for a message untill the timeout expires. However it could result in the 'null' return
from the Gateway method if the timeout has been reached before the actual reply was produced. It is also important to understand that
the reply message (if produced) will be sent to a reply channel after Gateway method invocation might have returned, so you must be aware of that
and design your flow with this in mind.
</para>
<para>
<emphasis>Downstream component returns 'null'</emphasis>
</para>
<para>
<emphasis>Sync Gateway - single-threaded</emphasis>.
If a component downstream returns 'null' and no <emphasis>reply-timeout</emphasis> has been configured, the Gateway
method call will hang indefinitely unless: a) <emphasis>reply-timeout</emphasis> has been configured or b)
<emphasis>requires-reply</emphasis> attribute has been set on the downstream component (e.g., service-activator)
that might return 'null'. In this case, the exception will be thrown and propagated to the Gateway.
<emphasis>Sync Gateway - multi-threaded</emphasis>. Behavior is the same as above.
</para>
<para>
<emphasis>Downstream component return signature is 'void' while Gateway method signature is non-void</emphasis>
</para>
<para>
<emphasis>Sync Gateway - single-threaded</emphasis>.
If a component downstream returns 'void' and no <emphasis>reply-timeout</emphasis> has been configured,
the Gateway method call will hang indefinitely unless <emphasis>reply-timeout</emphasis> has been configured 
<emphasis>Sync Gateway - multi-threaded</emphasis> Behavior is the same as above.
</para>
<para>
<emphasis>Downstream component results in Runtime Exception (regardless of the method signature)</emphasis>
</para>
<para>
<emphasis>Sync Gateway - single-threaded</emphasis>.
If a component downstream throws a Runtime Exception, such exception will be propagated via Error Message back to
the gateway and re-thrown.
<emphasis>Sync Gateway - multi-threaded</emphasis> Behavior is the same as above.
</para>
<para>
<important>
It is also important to understand that by default <emphasis>reply-timout</emphasis> is unbounded which means that
if not explicitly set there are several scenarios (described above) where your Gateway method invocation might
hang indefinitely, so make sure you analyze your flow and if there is even a remote possibility of one of these
scenarios to occur, set the <emphasis>reply-timout</emphasis> attribute to a 'safe' value or better off
set the <emphasis>requires-reply</emphasis> attribute of the downstream component to 'true' to ensure a timely response.
But also, realize that there are some scenarios (see the very first one)
where <emphasis>reply-timout</emphasis> will not help which means it is also important to analyze your message
flow and decide when to use Sync Gateway vs Async Gateway where Gateway method invocation is always guaranteed
to return while giving you a more granular control over the results of the invocation via Java Futures.
<para>
Also, when dealing with Router you should remember that seeting <emphasis>resolution-required</emphasis> attribute to 'true'
will result in the exception thrown by the router if it can not resolve a particular chanel. And when dealing with the filter
you can also set <emphasis>throw-exception-on-rejection</emphasis> attribute. Both of these will help to ensure a timely response
from the Gateway method invocation.
</para>
</important>
</para>
</section>
</chapter>